This invention relates to the preparation of hydrosilazane polymers by the reaction of disilazanes with trichlorosilane, HSiCl.sub.3. Such polymers are useful, when fired at high temperatures, in the formation of silicon nitride and silicon nitride-containing ceramic materials.
What is disclosed herein is a novel process to obtain novel hydrosilazane polymers which consists of contacting HSiCl.sub.3 with disilazanes in an inert, essentially anhydrous atmosphere while distilling volatile by-products.
As is well known in the art, halosilane monomers will react with ammonia and most organic compounds containing a primary or secondary amino group to give a variety of silazanes. For example, the reaction of trimethylchlorosilane and ammonia produces hexamethyldisilazane, a silazane monomer, while dimethyldichlorosilane and ammonia produce dimethylcyclic silazanes.
Silazanes in general have been academic curiosities for many years and a variety of such silazanes, including monomers, oligomers, cyclics and even low molecular weight resins and linear polymers have been prepared by a variety of methods. For example, L. W. Breed et al. in J. Org. Chem., 27, 1114(1962) report the formation of silazanes from the polymerization of sterically hindered silazane oligomers. In J. Polym. Sci. A, 2, 45(1964), cyclic trimer and tetramer silazanes are reported to be thermally cracked, using catalysts, to give linear polymers.
In contrast, fluids, rubbery polymers and resins prepared from CH.sub.3 SiCl.sub.3, (CH.sub.3).sub.2 SiCl.sub.2 and excess ammonia have been reported by Kruger et al. in J. Polym. Sci. A, 2, 3179(1964) and by Redl in "Silazane Polymer", ARPA-19, Advanced Research Projects Agency, October, 1965.
The patent literature also contains disclosures of the preparation of silazanes. Cheronis in U.S. Pat. No. 2,564,674 (Aug. 21, 1951) discloses the preparation of low molecular weight linear silazane polymers by the reaction of halosilanes with excess ammonia in a solvent solution. Bausma et al. in U.S. Pat. No. 3,809,713 (May 7, 1974) discloses a similar reaction scheme with the added modification of removing the by-produced solid ammonium halide using ethylene diamine.
Verbeek et al. in U.S. Pat. No. 3,853,567 (Dec. 10, 1974) and U.S. Pat. No. 3,892,583 (July 1, 1975) disclosed that mixtures of CH.sub.3 SiCl.sub.3 and (CH.sub.3).sub.2 SiCl.sub.2 can be treated with ammonia or organoamines to form materials that can be pyrolyzed to yield SiC/Si.sub.3 N.sub.4 ceramics.
In another segment of the prior art, the use of disilanes in the preparation of silazane polymers has been limited to the formation of relatively low molecular weight materials. In one example, Wannagat et al., Ang. Chem. 75(7), 345(1963), reported the reaction of tetramethyldichlorodisilane with gaseous ammonia to give a six-membered cyclic silazane, [(CH.sub.3).sub.2 SiSi(CH.sub.3).sub.2 NH].sub.2, rather than the expected linear silazane polymer. Hengge et al., Montash. Chem. 101(2), 325(1970), prepared dimethylamino substituted mixtures of disilanes from dimethylamine and a chlorine-containing disilane mixture obtained from the direct process for the preparation of chlorosilanes.
More recently, Gaul in U.S. Pat. Nos. 4,312,970 (Jan. 26, 1982) and 4,340,619 (July 20, 1982) has disclosed processes for preparing silazane polymers by reacting disilazane with either organochlorosilanes or chlorine-containing disilanes. The organochlorosilanes of U.S. Pat. No. 4,312,970 are described by the formula R.sub.n 'SiCl.sub.4-n where R' is a vinyl radical, an alkyl radical of 1-3 carbon atoms, or a phenyl radical and n has a value of 1 or 2. The chlorine-containing disilanes of U.S. Pat. No. 4,340,619 are described by the general formula (Cl.sub.a R.sub.b 'Si).sub.2 where R' is a vinyl radical, an alkyl radical of 1-3 carbon atoms, or a phenyl radical and where a has a value of 0.5-3, b has a value of 0-2.5, and the sum (a+b) is three. Ceramic materials prepared by firing the preceramic silazane polymers of Gaul at elevated temperatures under an inert atmosphere contained mainly silicon carbide as the crystalline phase. Silicon nitride, if found at all, was present in only minor amounts.
Seyferth et al. in U.S. Pat. No. 4,397,828 (Aug. 9, 1983) disclosed the preparation of relatively stable, liquid polymers containing silicon, nitrogen, and hydrogen which were formed by reacting dihydrodihalosilane such as H.sub.2 SiCl.sub.2 with ammonia in the presence of a solvent comprising an aliphatic ether, a chloromethane, or mixtures thereof. The method of Seyferth et al. produces rather large quantities of the troublesome by-product ammonium chloride. The liquid polymers were stable, under an inert atmosphere, for at least 7 days at 0.degree. C. but began to gel at room temperature after only 2 days. The liquid polymers of Seyferth et al., upon being fired to elevated temperatures under an inert atmosphere, yielded silicon nitride containing materials.
What has been newly discovered is the coreaction between trichlorosilane and disilazanes to give useful high molecular weight hydrosilazane polymers. The hydrosilazane polymers represent a significant advancement over prior art materials.